It would be desirable, for extravascular tumor diagnosis and therapy, to target an imaging or therapeutic compound selectively to the tumor via the bloodstream. In diagnostics, such targeting could be used to provide a greater concentration of an imaging agent at the tumor site, as well as reduced background level of the agent in other parts of the body. Site-specific targeting would be useful in therapeutic treatment of tumors, to reduce toxic side effects and to increase the drug dose which can safely be delivered to a tumor site.
Liposomes have been proposed as a drug carrier for intravenously (IV) administered compounds, including both imaging and therapeutic compounds. However, the use of liposomes for site-specific targeting via the bloodstream has been severely restricted by the rapid clearance of liposomes by cells of the reticuloendothelial system (RES). Typically, the RES will remove 80-95% of a dose of IV injected liposomes within one hour, effectively out-competing the selected target site for uptake of the liposomes.
A variety of factors which influence the rate of RES uptake of liposomes have been reported (e.g., Gregoriadis, 1974; Jonah; Gregoriadis, 1972; Juliano; Allen, 1983; Kimelberg, 1976; Richardson; Lopez-Berestein; Allen, 1981; Scherphof; Gregoriadis, 1980; Hwang; Patel, 1983; Senior, 1985; Allen, 1983; Ellens; Senior, 1982; Hwang; Ashwell; Hakomori; Karlsson; Schauer; Durocher; Greenberg; Woodruff; Czop; and Okada). Briefly, liposome size, charge, degree of lipid saturation, and surface moieties have all been implicated in liposome clearance by the RES. However, no single factor identified to date has been effective to provide long blood halflife, and more particularly, a relatively high percentage of liposomes in the bloodstream 24 hours after injection.
In addition to a long blood halflife, effective drug delivery to a tumor site would also require that the liposomes be capable of penetrating the continuous endothelial cell layer and underlying basement membrane surrounding the vessels supplying blood to a tumor. Although tumors may present a damaged, leaky endothelium, it has generally been recognized that for liposomes to reach tumor cells in effective amounts, the liposomes would have to possess mechanisms which facilitate their passage through the endothelial cell barriers and adjacent basement membranes, particularly in view of the irregular and often low blood flow to tumors and hence limited exposure to circulating liposomes (Weinstein). Higher than normal interstitial pressures found within most tumors would also tend to reduce the opportunity for extravasation of liposomes by creating an outward transvascular movement of fluid from the tumor (Jain). As has been pointed out, it would be unlikely to design a liposome which would overcome these barriers to extravasation in tumors and, at the same time, evade RES recognition and uptake (Poznansky).
In fact, studies reported to date indicate that even where the permeability of blood vessels increases, extravasation of conventional liposomes through the vessels does not increase significantly (Poste). Based on these findings, it was concluded that although extravasation of liposomes from capillaries compromised by disease may be occurring on a limited scale below detection levels, its therapeutic potential would be minimal (Poste).